A common problem in the construction of optical systems is how to align multiple components accurately, to provide correct optical performance. In free space diffractive and Fourier optical systems, correct placement of devices and components is critical, particularly when the tolerances involved can be at the micron level. Each component can require accurate alignment along the six main axes, denoted as the three linear translations x,y,z, and the three angular translations roll, yaw, pitch, considered relative to a central point. This criterion is extended if there are subsystems within the overall optical system, when global axes should be considered in addition to the local axes for each component.
Particular difficulty lies in the alignment of a plurality of optically coupled diffractive elements such as pixelated electro-optical micro-display arrays, commonly grouped under the term Spatial Light Modulators (SLMs). These can be Liquid Crystal on Silicon (LCoS) devices or mirror-based Micro-Electro-Mechanical (MEMs) devices.
In diffractive optical systems, diffractive elements such as SLMs are employed not as a means to produce images, but as a way of modulating coherent laser light of wavelengths comparable to the size of the pixels (for example 632 nm red visible laser light). By addressing patterns to the SLM pixel array, the light exiting the device may be shaped (in the case of holographic reproduction and optical tweezer applications), or may be used to input numerical data into an optical processing system. Examples of such systems include optical correlators (pattern recognition) and optical derivative functions (as the basis of larger equation solving systems), such as those proposed in published patent application numbers US2010085496 and US2006050986. By addressing specific patterns such as zone plates, Fresnel lenses and phase ramps, SLMs may also be used in place of traditional focussing and beam steering elements (lenses, mirrors), as proposed in PCT/GB2013/051778.
Using these principles large component count diffractive optical systems may be realised using SLMs to both input data into the optical system, as well as to direct and focus the light as required. This has the significant advantage of the optical alignment becoming a software task, rather than a hardware task, since the patterns may be dynamically adjusted once the initial physical alignment is completed—and furthermore the system may be reconfigured to form other optical systems. However, the issue of how to physically align the SLMs remains. When considering that an optical partial differential equation solver system may need to contain over 200 SLMs, the difficulty in aligning such a quantity of components becomes apparent.